Generating power from nuclear fission utilizes a process in which fissionable nuclei of certain elements, for example uranium 235 (235U), uranium 233 (233U), or plutonium 239 (239Pu) undergo spontaneous fission or fission stimulated by absorption of a neutron. During fission, a nucleus splits into two smaller nuclei and a number of free neutrons. Neutrons produced in a fission event typically have large kinetic energy, typically of order MeV, and are called fast neutrons. In a conventional nuclear reactor, a critical core typically includes fuel rods, or pins, containing fissionable nuclei. The fuel pins are arranged within a matrix of a material that decreases the kinetic energy of neutrons. This process is called moderation. A critical core is capable of self-sustained fission and is called a nuclear reactor.
Stimulated fission of a 235U nucleus has maximum probability for an incident neutron of low energy, typically of order eV, called a thermal neutron . . . Reactors using 235U fission utilize low-atomic-weight materials, such as water or carbon, as moderators because fast neutrons scattering from such light nuclei quickly lose kinetic energy and become available to stimulate fission. The fuel pins and moderator in a conventional 235U-fueled core are arranged so as to sustain an equilibrium in which just enough neutrons are produced in fission to stimulate more fission. Such an arrangement is called a critical pile. The condition of equilibrium must be stabilized by insertion or removal of additional rods of a material whose nuclei have large probability to capture neutrons (control rods). The insertion and removal of control rods can thus be used to maintain the neutron gain, or criticality, of the pile at the precise value of one; this situation is called a critical reaction. If too many neutrons are absorbed, the rate of fission decreases exponentially with time and the core shuts down. If too few neutrons are absorbed, the rate of fission increases exponentially with time and the core explodes.
A critical fission core can be used to generate a large amount of heat, the heat used to generate steam, and the steam used to drive electric generators. Water-moderated 235U-fueled fission reactors are commonly used to generate electric power.
In addition to stimulated fission, neutrons may be captured on certain heavy nuclei, for example uranium 238 (238U) and thorium (232Th), and through a sequence of such neutron capture and radioactive decay produce a fissionable nucleus such as 239Pu (from 238U) or 233U (from 232Th). This process is called breeding, and provides a mechanism by which the process of stimulated fission can actually produce additional fissionable nuclei within the core material
Breeding can also lead to the formation of yet-heavier elements, beyond plutonium in the periodic table. Such elements are called minor actinides. Examples of minor actinides include neptunium (Np) and americium (Am). The minor actinides present a significant problem for safety of nuclear power, because they are produced in significant quantity in thermal reactors and they are the only elements that have radioactive decay half-life greater than a century and less than a million years. For example americium (241Am) has a half-life of 432 years; 243AM has a half-life of 7,370 years. For that reason they present a serious problem for disposal of spent nuclear fuel.